A number of techniques are known to those skilled in the art for inducing fusion between cells. These techniques include chemical fusion employing polyethylene glycol, use of biological fusogens such as viruses or viral proteins, and electrofusion of cells in suspension. Fusion by chemical means or via biological fusogens has certain limitations including the presence of chemical or biological contaminants inherent to the technique, resistance to fusion exhibited by some cell types, low efficiency, and cytotoxicity.
There are several advantages in producing cell hybrids by electrofusion. For example, fusion parameters can be easily and accurately electronically controlled to conditions depending on the cells to be fused. Further, electrofusion of cells has shown to the ability to increase fusion efficiency over that of fusion by chemical means or via biological fusogens. Electrofusion is performed by applying electric pulses to cells in suspension. By exposing cells to an alternating electric field, cells are brought close to each other in forming pearl chains in a process termed "dielectrophoresis alignment". Subsequent higher voltage pulses cause cells to come into closer contact, reversible electropores are formed in reversibly permeabilizing and mechanically breaking down cell membranes, resulting in fusion.
However, electrofusion of cells in suspension has certain limitations. The field strength of the electric pulse required to induce reversible electropores in permeabilizing membranes is cell size dependent. Thus, it is difficult to efficiently electrofuse cells of different sizes simultaneously. In such instances, larger cells are typically damaged by a given field strength required to induce reversible electropores in permeabilizing membranes of smaller cells. Further, it is known generally that the lower the medium conductivity, the more effective is the dielectrophoresis alignment for cells in suspension. A medium of higher conductivity may reduce the threshold field strength to induce mechanical breakdown of membranes. Thus, medium typically used for electrofusion of cells in suspension has a conductivity which is not optimal for both dielectrophoresis alignment and inducement of mechanical breakdown of membranes.
It would be desirable to provide a method and apparatus to fuse cells of heterogenous sizes, or to transfer macromolecules into target cells, with high efficiency and in the presence of a variety of pulse medium.